The present invention relates generally to crystal growing apparatus used in growing monocrystalline ingots, and more particularly to an electrical resistance heater for use in such a crystal growing apparatus.
Single crystal silicon, which is the starting material for most semiconductor electronic component fabrication, is commonly prepared by the so-called Czochralski ("Cz") method. The growth of the crystal is most commonly carried out in a crystal pulling furnace. In this method, polycrystalline silicon ("polysilicon") is charged to a crucible and melted by heat generated by a crucible heater surrounding the crucible side wall. A seed crystal is brought into contact with the molten silicon and a single crystal ingot is grown by slow extraction via a crystal puller. After formation of a neck is complete, the diameter of the crystal ingot is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached.
The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the pull rate and heat supplied to the crucible. When the diameter becomes small enough, the ingot is then separated from the melt.
Crucible heaters used for melting silicon in the crucible are typically electrical resistance heaters in which an electrical current flows through a heating element constructed of a resistive heating material (e.g., graphite). The resistance to the flow of current generates heat that radiates from the heating element to the crucible and silicon contained therein. The heating element comprises vertically oriented heating segments of equal length and rectangular cross-section arranged in side-by-side relationship and connected to each other in a serpentine configuration. That is, adjacent segments are connected to each other at the tops or bottoms of the segments in an alternating manner to form a continuous electrical circuit throughout the heating element. The heating power generated by the heating element is generally a function of the cross-sectional area of the rectangular segments and the current input to the heating element. Input current to the heater is typically limited to about 60 volts to inhibit arcing between the heater and any surrounding grounding surfaces of the crystal puller.
Although the conventional apparatus used for growing single crystal silicon according to the Czochralski growth method has been satisfactory for growing crystal ingots useful in a wide variety of applications, further improvements in the quality of the semiconductor material are desirable. As the width of integrated circuit lines formed on the semiconductor material continue to be reduced, the presence of defects in the crystal become of greater concern. A number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, because of the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects, which are known as vacancies and self-interstitials.
One important measurement of the quality of wafers sliced from a singlecrystal ingot is Gate Oxide Integrity ("GOI"). Vacancies, as their name suggests, are caused by the absence or "vacancy" of a silicon atom in the crystal lattice. When the crystal is pulled upward from the molten silicon in the crucible, it immediately begins to cool. As the temperature of the crystal ingot descends through the temperature range of 1150 .degree. C. down to 1050 .degree. C., vacancies present in the ingot tend to migrate out toward the outer surface of the ingot or agglomerate together within the ingot. These agglomerations are manifested as pits within the surfaces of the wafers sliced from the crystal ingot.
Silicon wafers sliced from the ingot and manufactured according to conventional processes often include a silicon oxide layer formed on the surface of the wafer. Electronic circuit devices such as MOS devices are fabricated on this silicon oxide layer. Defects in the surface of the wafer, caused by the agglomerations present in the growing crystal, lead to poor growth of the oxide layer. The quality of the oxide layer, often referred to as the oxide film dielectric breakdown strength, may be quantitatively measured by fabricating MOS devices on the oxide layer and testing the devices. The Gate Oxide Integrity (GOI) of the crystal is the percentage of operational devices on the oxide layer of the wafers processed from the crystal.
It has been determined that the GOI of crystals grown by the Czochralski method can be improved by increasing the amount of time a growing ingot dwells in the temperature range above 1000.degree. C., and more particularly in the range of 1150.degree. C.-1050.degree. C. If the ingot cools too quickly through this temperature range, the vacancies will not have sufficient time to agglomerate together, resulting in a large number of small agglomerations within the ingot. This undesirably leads to a large number of small pits spread over the surfaces of the wafer, thereby negatively affecting GOI. Slowing down the cooling rate of the ingot so that its temperature dwells in the target temperature range for a longer period of time allows more vacancies to move to the outer surface of the ingot or form large agglomerations within the ingot. The result is a small number of large agglomerations, thereby improving GOI by decreasing the number of defects present in the surface of the wafer upon which the MOS devices are formed.
To this end, co-assigned U.S. application Ser. No. 60/090,798, filed Jun. 26, 1998, discloses a second, or upper electrical resistance heater for use in a crystal puller used for growing monocrystalline silicon ingots according to the Czochralski method which is sized and shaped for placement in the housing of the crystal puller generally above the crucible in spaced relationship with the outer surface of the growing ingot. The upper heater radiates heat to the ingot as the ingot is pulled upward in the housing relative to the molten silicon. This upper heater is constructed in a manner similar to conventional crucible heaters in that it comprises vertically oriented heating segments arranged in side-by-side relationship and connected to each other to form an electrical circuit. The heater has opposing mounting brackets extending upward from the segments for mounting the upper heater on the wall of the upper pull chamber. The upper heater is preferably capable of radiating heat at a temperature in the range of 1000.degree. C.-1100.degree. C. The heating segments may vary in length to define a profiled heater in which the heater radiates more heat to the crystal ingot at the top of the heater than at its bottom.
Over time, it has become desirable to further lengthen the heating segments of the upper heater to maintain the temperature of the growing ingot above 1,050.degree. C. for as long as possible. However, as the lengths of the heating segments increase within the limited radial space available in the crystal puller housing, the segments become more fragile. The heater would thus become less robust in the radial direction and subject to a greater risk of damage caused by bending, particularly during handling such as installation in or removal from the crystal puller housing. Increasing the cross-sectional area of the segments would certainly increase the radial bending stiffness of the segments, but would undesirably reduce the electrical resistance of the segments, thereby reducing the total power output of the heater for a predetermnined input current.